Adiponectin is an adipocyte-derived hormone that exhibits insulin sensitizing properties, although the use of adiponectin as a therapeutic treatment is infeasible due to its high circulating serum concentration, short half-life, complex molecular structure, and the discovery of adiponectin resistance. The beneficial effects of adiponectin are mediated by the adaptor protein APPL1, and its molecular signaling characteristics are of interest, as they are potential therapeutic targets. The identification of a novel APPL1 isoform, APPL1 short form (APPL1-sf) is of significant interest, as it has demonstrated its ability to down-regulate hepatic adiponectin signaling and has the potential to antagonize APPL1 function by interacting with adiponectin receptors, AdipoR1 and AdipoR2, suggesting it also mechanistically mediates insulin sensitivity through regulating adiponectin signaling. The hypothesis is that APPL1-sf is a negative regulator of adiponectin signaling in liver and the protein exerts its roles by antagonizing APPL1 function. The specific aims are 1) to determine how APPL1-sf inhibits hepatocyte adiponectin signaling, 2) to determine the role of APPL1-sf in regulating insulin sensitivity in liver and in vivo Research Design: The identification of the novel APPL1-sf is significant for studying the molecular mechanisms that regulate adiponectin signaling and insulin sensitivity, as it was found to act as a negative regulator of hepatic adiponectin signalin. For Aim 1, we will first examine the subcellular localization of APPL1-sf and its co-localization with adiponectin receptors (AdipoR1 and AdipoR2) in the presence or absence of adiponectin. We will then examine the interaction between endogenous APPL1- so and AdipoR1/R2, AdipoR1 and AdipoR2 will be immunoprecipitated with specific antibodies to these proteins. Lastly, we will determine the effect of over-expressing or knocking down APPL1-sf on adiponectin signaling in hepatocytes. For Aim 2, we will first examine whether APPL1-sf levels in the liver are affected by diet-induced obesity in vivo. Next, we will determine the role of APPL1-sf in adiponectin function in hepatocytes by analyzing its effect on hepatic glucose production. Lastly, we will evaluate glucose homeostasis in liver-APPL1-sf knock down mice and their control mice in vivo. The goal of this proposed study is to characterize the role of APPL1-sf in regulating insulin sensitivity though adiponectin signaling. By elucidating the molecular mechanisms of APPL1-sf in mediating insulin sensitivity, potential therapeutic targets may be identified and mimics designed to treat insulin resistance.